TPMT (Thiopurine S-Methyltransferase): TPMT is an enzyme that metabolizes thiopurine drugs like azathioprine, mercaptopurine, and thioguanine, which are used to treat autoimmune diseases and cancer. Genetic variations in TPMT influence enzyme activity, affecting drug metabolism and the risk of toxicity. People with low TPMT activity have a higher risk of severe bone marrow suppression. TPMT testing helps tailor thiopurine dosages to minimize side effects and enhance treatment safety.